Engine life may be shortened if any of several critical parameters are exceeded during operation. For this reason maximum datum levels are set and the relevant parameters are monitored and fed back to an engine management system which maintains engine operation within the limits set by the datum levels. Turbine temperature is one such critical parameter. In one example it is measured by monitoring turbine gas stream temperature by means of thermocouples, for example. In another an optical pyrometer is used to measure the temperature of the turbine blades by infra-red emission.
In the example described below the limiting effect on jet engine life is turbine blade temperature (TBT). An engine management system continuously measures the TBT and controls the fuel burn so as to limit the TBT to a particular value according to prevailing circumstances. The datum limit value of TBT chosen is a compromise between performance and engine life. For a given required engine life there is a particular value of TBT for particular circumstances and thus a corresponding maximum available thrust. Temporary increased thrust is available by allowing an increase in TBT but the cost is extremely severe in terms of reduction of engine life, the reduction being dependent upon the extent of the TBT increase and the time it prevails.
The engine management system generally includes a digital electronic control unit (DECU) which receives input from a pilot control input which represents a demand for power; a TBT sensor and ambient pressure and temperature sensors. The DECU is programmed to control the fuel feed to the engine in order to meet the pilot power demand subject to the datum limit of TBT for the prevailing circumstances not being exceeded.
Some engines are fitted with a reheat system by operation of which engine thrust may be increased, usually, for short periods of time. The TBT limit may be raised for reheat operation, any such change is implemented by reheat selection independently of any other manual input. Provision may be made for the pilot to manually override the limiter in certain circumstances.
There are special circumstances other than emergencies where operating advantages could be achieved by over-riding the normal control characteristics of the DECU temporarily. The present invention provides an engine management system capable of providing enhanced performance under special circumstances without the risk of pilot abuse.